The Japanese freshwater crab Geothelphusa exigua Suzuki and Tsuda, 1994, is sympatric with Geothelphusa dehaani (White, 1874) in riverine habitats in Ohsumi Peninsula, Kagoshima Prefecture, southern Kyushu, Japan. We examined a number of biological characteristics of both crabs in two rivers in Ohsumi Peninsula to elucidate the factors maintaining their sympatric distribution. Results showed few biological differences between the two species, except for maximum body sizes and habitat preference. Geothelphusa exigua is smaller than G. dehaani in the maximum body size and lives mainly in water (aquatic), whereas G. dehaani may live both in water and on land (amphibious).